build(deps): bump actions/download-artifact from 3 to 4

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 3 to 4.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v3...v4)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/release.yml

@@ -66,11 +66,11 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Download bithesis.pdf
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: bithesis
       - name: Download `*.cls`
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: cls
       - name: Copy manual to the template folder.
@@ -102,7 +102,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Download *.cls
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: cls
       - name: Upload ${{ matrix.cls }}.cls to release
@@ -138,7 +138,7 @@ jobs:
 
       - uses: actions/checkout@v4
       - name: Download PDF(s)
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: bithesis
 
@@ -160,7 +160,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Download bithesis.pdf
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: bithesis
       - run: make pkg-only